Interest in the physiological activities and potential clinical uses of the pineal hormone melatonin (N-acetyl-5-methoxytryptamine) has grown enormously in recent years. The hormone has been implicated in the regulation of phenomena as diverse as the sleep-wake cycle, depression, sexual urge, neuroimmunomodulation, mammary cancer, etc. Such multiplicity of actions in different tissues suggests a multiplicity of receptors for the same molecule. We plan to test this possibility by photoaffinity labeling with azido analogs of melatonin in which the azido function is found in different positions in the molecule. Identification of the protein sequence carrying the ligand will then determine if the receptor proteins are identical or not. To this end, the N-acetyl group of melatonin has been replaced by the azidoacetyl group. Efforts will now be made to radioiodinate the new analogue. We have found that the universally used "iodogen" method for radioiodination suffers from very low yields and often leads to competitive chlorination. We have developed an alternative method in which hydrogen peroxide is used to oxidize iodine ion in a two-phase system. This method not only avoids the expensive iodogen and the danger of chlorination, but also provides considerably improved yields of iodinated material. Previous efforts to produce 2-fluoro analogues of indoles, by exchange of other halogens or by direct fluorination, were minimally successful. We now have found that commercial morpholinosulfurtrifluoride will replace the carbonyl group of an oxindole with fluorine, providing the first and highly practical route to 2-fluoro derivatives of bioindoles. These compounds are expected to serve as irreversible affinity labels at brain receptors. Structure-activity studies show that bulky, lipophilic substituents at C-2 improve the binding of melatonin. Methods which were highly successful in producing 2-alkylhistidines and histamines are now being applied to produce 2-alkyl derivatives of bioindoles.